Astronomical Imaging: Catching up to the 1950s

Michael Richmond
Apr 13, 2013

We live in a world full of electronic gadgets.
Smartphones, big-screen television sets,
GPS receivers.
Every year, they seem to get bigger and better.

Most astronomers today, both professional and amateur, use
electronic detectors to take astronomical
images.
Is there any doubt that our current cameras are
bigger and better than any used by scientists
decades ago?

YES,
there is doubt.

Let me try to convince you that, if we choose one reasonable
manner of measuring the quality of a detector,
it is only now -- or, perhaps, in the past five years --
that modern electronic detectors have finally
caught up to the good old photographic plate.

The eye focuses light rays onto light-sensitive
cells in the retina, along the back surface of the eyeball.
When enough photons strike a set of these cells,
a signal is sent through the optic nerves to the brain.

quantum efficiency

integrate?

permanent?

linear?

ease of use

size

Eye

6 %

no

no

no

simple

36 sq. mm

In the middle of the nineteenth century,
chemists and artists figured out that it was possible
to focus light onto a specially prepared
film of material which could record the
image permanently.
The early photographic techniques had very
low sensitivity to light -- requiring long
exposure times to build up a good picture
(that's why the people in early photographs
are posed so stiffly).
The first photograph of the Moon,
a daguerreotype,
required Henry Draper to expose the plate for 20 minutes!

Image courtesy of Greenwich Village History

As the decades passed, chemists developed
materials which were much more sensitive to light,
enabling photographers to take images with
exposure times of one second or less.
Special emulsions were developed for astrophotography
which were especially sensitive to low light levels.
Do you recognize any of these names?

However, even the best photographic emulsions record
only a tiny fraction of all the light that strikes them.
Pay attention to the second of these graphs.

As you can see from the films which we are passing
out to the audience now,
another big advantage film offered to astronomers
was its size:
one could (with very careful fabrication techniques)
spread photographic emulsion over very large
areas, over a backing of glass (plates) or plastics (film).
Opticians like
Bernhard Schmidt designed special telescopes which
could project sharp images covering wide areas of
the sky onto large photographic plates.

In 1970, scientists at Bell Labs were thinking about
ways to store information in solid-state memory.
They came up with the idea of a silicon chip
divided into an array of small regions, then moving
electric charges into and out of the array.
By varying voltages applied to small sections
of the chip, the user could couple the charge
to specific regions. These chips became known
as charge coupled devices, or CCDs.

It turns out that silicon has many interesting
properties: one is its ability to convert individual
photons of light into individual electrons.
People soon realized that with a little preparation,
CCDs could be used to convert light into an image,
and then to transfer that image digitally to a computer.
Because the devices were novel and expensive, the first
applications were in space:
astronomers suggested using CCDs for the cameras
aboard the Galileo mission to Jupiter, and on the
orbiting telescope we now call
the Hubble Space Telescope.
As years passed, many companies started to fabricate
CCDs for industrial applications and the prices dropped.
Even ground-based astronomers -- at first professional,
but then amateur -- were able to afford these new
imaging devices.

Q: When did CCDs really take over in the
community of professional astronomers?
A. around 1970
B. around 1980
C. around 1990

Watch as the fraction of papers with the word
photographic in the abstract slowly decreases ....

... while the fraction with CCD in the
abstract increases.

These silicon-based detectors had two big advantages
photographic film

the sensitivity was much higher

the response was linear

The "sensitivity" part is easy to understand,

but what does the "linear" part mean?
It means that if one exposes the detector for twice as
many seconds, one OUGHT to record a signal twice as large.

You may have noticed that size is listed
in the tables above.
It might seem to be a simple matter -- a bigger detector
will be able to measure light over a wider field,
so bigger must mean better.
Right?

Well, almost. What we really want is the ability
to record as much fine detail as possible.
If two detectors can distinguish details at the
same physical scale, then, sure, the bigger one will
capture more information.
But what if they differ in the way they respond
to light?

Consider the two detectors shown below.
Detector A is twice as large as detector B.

Q: Which will record more information?
Detector A Detector B

Let's find out. Below are two pictures of the same region
of the Moon, taken with the same optical setup, but two
different detectors.

What really matters is a combination of
the size of the detector AND the size of one of its
resolution elements.
A resolution element (sometimes called a "pixel")
is simply the smallest region
of the detector which can respond as a single unit
to incoming light.
We need to compute this combination:

The chip is square, so the number of resolution elements
is 4096 x 4096 = 16 million, if we round off a bit.

Now, photographic emulsion (like CCDs) comes in many varieties,
each of which has somewhat different properties.
The emulsions used in astronomical applications
such as the Palomar Observatory Sky Surveys had relatively
coarse grains. The smallest dark spot which forms in
response to light consists of a small number of grains
and is roughly 5 to 10 microns in diameter.
If we adopt the larger size for our calculation,
then a plate 14 inches on a side has

Using this value, we would estimate the eye to contain
roughly 6,000 x 6,000 = 36 million resolution elements,
similar to a very large modern CCD.

The bad news

Alas, things aren't so rosy inside the human eye.
To begin, the light-sensitive cells, both rods
and cones, aren't evenly distributed.
Each type is more concentrated near the center
of the eye;
the very central region, called the fovea,
has a very high concentration of cones.
When we need to see the fine detail in some area,
we move our eyes so that that area falls on the fovea.
As a consequence, rods are more sparsely placed
in the outer regions of the retina, which make up
most of its area.

Moreover, several rod cells will send their signals
to a single neuron, especially in the outer
regions of the retina. So, even though each rod
cell may record the light which strikes it alone,
the brain will receive a single signal which
represents the mix of responses from a number of
cells. This reduces the number of resolution
elements considerably.

The total number of axons in the optic nerve
is roughly 1 million, which can serve
as an estimate of the number of resolution elements
in the eye. That's quite a bit smaller than one
would expect from the analysis above, and it
definitely moves the eye into third place
in the "number of resolution elements" contest.

This question also has an obvious winner: the CCD.
If one takes a picture with a fixed exposure length,
the CCD will record stars and galaxies which are
much fainter than those seen on a photographic plate
or the human eye.
To put it another way, in order to record the same
level of detail for one particular star,
we must expose for much longer with a photographic plate
than with a CCD.

photograph 3 percent of light recorded
CCD 70 percent of light recorded
eye 6 percent of light recorded

If we are interested in a small region of the sky --
say, the area around McNeil's Nebula -- then
the CCD is a better choice.
Compare these views taken with a LONG exposure
on film (on the left) and a SHORT exposure on a CCD
(on the right).
In order to reach the same depth, we must expose
the film for roughly 17 times longer.

On the other hand, if we are interested in a wide area
on the sky,
film is a better choice.
The picture above shows nearly the entire
the entire CCD image;
but it is only a small portion of the entire
photographic plate.

A REALLY small portion of the entire photographic plate ...

In order to cover the same area on the sky with a CCD,
we would need to take many more exposures, shifting
the telescope a bit each time:

So, in the battle between a photographic plate and a
single CCD chip,
we must compare the longer exposure time for
the plate against the multiple exposures required
for the CCD.
If we use the numbers from
our table above ,
we can compute an overall efficiency
for each detector:
it's the size (large is better)
multiplied by the sensitivity (again, large is better).

Surprisingly enough,
when it comes to recording large areas of the
sky, the venerable photographic plate can be
more efficient than one CCD.

Yes, yes, I've ignored the factors of linearity
and dynamic range, which favor the CCD.

But -- what if we put several CCD chips into
a single camera to create a mosaic?

One of the first mosaic cameras was created for the
Sloan Digital Sky Survey
(SDSS for short).
The design was a bit unusual:
the CCDs were arranged into 6 columns, each of which
had a series of 5 chips with different filters.
Instead of tracking the stars exactly,
the SDSS telescope would move at a slightly non-sidereal
rate in a carefully calculated direction;
as a result, stars would drift slowly across the
camera, along these columns.
In less than ten minutes, the camera would collect
an image of each object in the five filters,
allowing scientists to measure the color
of celestial objects.

name

# of CCDs

# of pixels

area

first* use

SDSS

30

126 million

72,600 sq. mm

1998

* first regular operation

The Canada-France-Hawaii Telescope (CFHT),
built in 1979,
has a mirror 3.6 meters in diameter.
It sits atop Mauna Kea and is designed to
perform well in the infrared as well as the optical.
In 2003, astronomers installed a very large camera
called
MegaCAM
in order to carry out large-scale surveys of
the sky.
You can examine the data collected by this instrument
by visiting the
CFHT Legacy Survey archive site.
You can also
browse the CFHT Deep Field #1
with your browser -- there are just too many pixels
to show on the screen at once!

name

# of CCDs

# of pixels

area

first* use

SDSS

30

126 million

72,600 sq. mm

1998

MegaCAM

36

340 million

62,000 sq. mm

2003

* first regular operation

OmegaCAM
is a 32-chip mosaic designed for use on the
wide-field VLT Survey Telescope,
a 2.6-m telescope in the mountains of Chile.
Each of its chips has 2K-by-4K pixels,
so the entire instrument
contains 16K-by-16K = 268 megapixels.
The total area of the camera is
260 x 260 mm, which translates into
1 x 1 degree on the sky.

name

# of CCDs

# of pixels

area

first* use

SDSS

30

126 million

72,600 sq. mm

1998

MegaCAM

36

340 million

62,000 sq. mm

2003

OmegaCAM

32

268 million

60,000 sq. mm

2011

* first regular operation

The final stop on our tour of mosaic CCD cameras
is one which is still in its testing phase.
Hyper-SuprimeCam
-- which takes over for regular
old SuprimeCam -- will sit at the focus of the
Subaru 8.3-meter Telescope on Mauna Kea.
By packing over 100 specially designed CCD
chips together
into an area over 2 feet wide, scientists will be able to
cover a field of view roughly 1.5 degrees
wide in a single shot.

name

# of CCDs

# of pixels

area

first* use

SDSS

30

126 million

72,600 sq. mm

1998

MegaCAM

36

340 million

62,000 sq. mm

2003

OmegaCAM

32

268 million

60,000 sq. mm

2011

Hyper-SuprimeCam

116

973 million

210,000 sq. mm

2013 (?)

* first regular operation

So, we have finally reached a point at which electronic
detectors have matched photographic plates in their
ability to record large volumes of information in
a single exposure.
Recall from
our earlier calculations
that the large photographic plates used
in the 1950s for the Palomar Observatory Sky Survey
had roughly 1.3 billion resolution elements.
The Hyper-SuprimeCam mosaic CCD camera has almost
as many -- 0.97 billion -- and records
much more of the light which strikes it.

Basic CCD imaging
by Pedro Re (I think) is a nice PowerPoint presentation on the
history and workings of CCDs

One of the big new mosaic CCD cameras I
did not mention is the
DECam,
part of the then
Dark Energy Survey.
The camera will take very
deep images of large regions of the sky
in order to measure the geometry of the
universe.